Throat mix burner

ABSTRACT

A throat mix burner providing efficient air and fuel mixing, flame stability, high turndown ratio and safety. These results are achieved through selected size relationships among the various dimensions of the burner combustion chambers.

United States Patent Hemsath et al.

[4 1 Jan. 21, 1975 THROAT MIX BURNER inventors: Klaus H. Hemsath,Sylvania; Arvind C. Thekdi, Toledo, both of Ohio Assignee: Midland RossCorporation, Cleveland, Ohio Filed: May 8, 1974 Appl. No.: 467,928

Related US. Application Data Continuation of Ser. No. 313,689, Dec. 11,1972, abandoned.

US. Cl 43l/353, 431/174, 431/263,

239/425 Int. Cl. F23d 15/02 Field of Search 431/174, 177, 181, 187,

[56] References Cited UNITED STATES PATENTS 1,975,033 9/1934 Wolff239/4165 2,368,370 1/1945 Maxon. 431/187 3,154,134 10/1964 Bloom239/4165 3,676,048 7/1972 Sellors et al. 431/187 PrimaryExaminer-Carrol1 B. Dority, Jr. Attorney, Agent, or Firm-Peter Vrahotes;Frank J. Nawalanic [57] ABSTRACT A throat mix burner providing efficientair and fuel mixing, flame stability, high turndown ratio and safety.These results are achieved through selected size relationships among thevarious dimensions of the burner combustion chambers.

1 Claim, 4 Drawing Figures SHEET 2 BF 2 PATENTEBJANZI ms FIG.3

THROAT MIX BURNER This is a continuation, of application Ser. No.313,689, filed Dec. 11, 1972, now abandoned.

BACKGROUND OF THE INVENTION Modern industrial heating processes requireprecise and accurate control of a number of variables such as furnacetemperature and atmosphere composition. In most cases, the operatingcharacteristics of the burner used in the furnace, or the processreactor, have a significant effect on these variables. It isadvantageous to use a burner which can operate under a variety ofconditions such as firing rate, air-fuel ratio, type of fuel and thelike. In addition to this, the burner must have a reliable ignition andflame supervision system to avoid mishaps. When all of theserequirements are considered, a good burner should possess the followingcharacteristics:

1. Efficient mixing of air and fuel, and complete combustion of theair-fuel mixture within the combustion chamber of the burner.

2. Flame stability without excess and erratic noise.

3. Wide range of turndown while maintaining a preset air-fuel ratio.

4. The designed flame shape should be maintained within a specifiedrange of the burner operation.

5. The burner parts should not overheat, deform, or be damaged during anextended period of burner operation.

6. Minimum amount of pollutants such as CO, NO, and other noxious gases,which originate from the combustion process.

7. Dependable and safe ignition system.

8. Foolproof and positive flame supervision system.

The above and other characteristics, advantages and objects are achievedthrough the present invention as will be readily apparent from thefollowing description, reference being made to the accompanying drawingwherein:

FIG. 1 is a longitudinal sectional view through a throat mix burnerembodying the principles of this invention;

FIG. 2 is a cross-sectional view taken along line 2-2 of FIG. 1;

FIG. 3 is an end view of the apparatus shown in FIG. 1; and

FIG. 4 is a detailed view showing the means for igniting the burner inFIG. 1.

SUMMARY OF THE INVENTION The burner of this invention is of the balancedpressure type which uses a ratio control device, such as a zeroregulator, to maintain the desired air-fuel ratio. Both the air and thegas travel axially into a first chamber portion of the throat of theburner with the air forming an annulus about the gas supply means. Thegas is directed radially into this air annulus after which the gasestravel down the throat into a second chamber throat portion havingdiverging walls. From the second chamber the gases go into a thirdportion having a right cylindrical shape. The geometrical proportions ofthe three chambers relative to one another are critical and uponselection of the proper proportions the objects of this invention areachievable.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing, aburner which incorporates the principles of this invention is showngenerally at 10. The burner has a generally annular block 12 made of arefractory such as high alumina castable. Extending through the burneris a preconfigured bore or throat having a first right cylindricalchamber 14 at the upstream end thereof. At the downstream end of firstchamber 14 is a shoulder or step 16 which is part of a conically shapedchamber 18. Downstream from the conically shaped chamber 18 is a shallowsecond right cylindrical chamber 20.

The block 12 is received within a metallic holder 24, which holder hasaxially extending flanges 26, that are generally annular, and abackplate 28. The backplate 28 extends radially beyond the annularflanges 26 and has a central opening'30 therein, which opening iscoaxially aligned with first chamber 14. The backplate also has aplurality of outlying openings 31 which provide means for securing theburner 10 to a furnace wall (not shown). The block 12 is held securelywithin the holder 24 as by cement 32.

A burner casting 34 is rigidly secured to the backplate 28 as by boltassemblies 35. The burner casting 34 has an axial opening 36 thatreceives a gas line 38. A zero regulator 39 is connected to the gas line38. Additionally, the casting 34 has a radial opening 40 that receivesan air line 42. Air is supplied to the air line 42 by an air header 41,there being a back load line 43 providing communication between the zeroregulator 39 and the air header 41. In essence, the combination of theblock 12, the backplate 28 and the casting 34 may be considered ahousing for the burner to support other components.

Received within the burner casting 34 and extending into first chamber14 is a tubular nozzle 44 which defines, in combination with the block12 interior walls, an annulus 45. The end of the nozzle 44 receives asolid disc 46 and the nozzle has a plurality of peripherally spacedopenings 48 located near its downstream end. A generally star-shapeddisc 49 is received within the opening 30 to properly position thenozzle 44.

In order to provide ignition means for the mixture of gas and air in thethroat, the block 12 and backplate 28 are provided with an angularopening 50 which receives a spark plug 52 whose electrodes extend tofirst chamber 14. It will be understood that although the ignition meansis shown extending into first chamber 14, it may also be located at thestep 16 or conical chamber 18. Additionally, a sight glass 54 isincluded to provide visual examination and supervision of the flame. Ascan be seen from the drawing, the location of the sight glass 54 is suchthat light from the ignition means does not reach the view angle of thesight glass 54. The flame may be controlled automatically by providing acontrol means 56 such as an ultraviolet detector at the sight glass orby use of a flame rod.

In operation, being a nozzle mix burner 10, the combustion air and fuelare supplied separately to the burner. The fuel is admitted into theburner 10 from the nozzle 44 which may have radial holes 48 or an axialhole in the plate 46, although radial holes, as shown, are preferable.The size and number of radial holes 48 depend on the type of fuel,available fuel pressure and the size of the burner 10. The radial holes48 promote better mixing of air and fuel, and give a short, intense,nonluminous flame. In cases where a relatively long flame is required,the gas nozzle 44 may have an axial hole, rather than the radial holes.The size of such an axial hole, again, would depend on the factorsmentioned above.

Combustion air is introduced through the annulus area 45 between the gasnozzle 44 and wall of first chamber 14. The area of the air passagedepends on the available air pressure drop, the rated burner capacityand the air density. The combustion air creates a jet pump effect.

The air and fuel then travels into and through first chamber 14. Theratio of first chamber length (Lt) to throat diameter (Dt) should befrom 0.5 to 10.0, in order to obtain satisfactory burner performance.Preferably the ratio of Lt to Dt should be from 6.5 to 8.5. In practice,the ratio is determined by the required burner block length and theposition of the burner ignition means 52. The distance between the endof the fuel nozzle 44 and the position of the ignition device 52 canvary from at least one throat diameter (Dt) to that permitted by firstchamber length (Lt).

First chamber 14 is followed by the step 16 in the burner block 12. Thestep 16 is used to stabilize the flame by providing recirculation of theair-fuel mixture and the products of combustion. The fresh air-fuelmixture mix during the recirculation, thereby increasing the air-fuelmixture temperature until the mixture is ignited. The ratio of the stepdiameter (Ds) to first chamber diameter (Dt) should be within the rangeof 1.15 to 1.5. After the step 16, the burner block 12 is shaped in theform of a conical chamber 18, which is followed by the shallow shortright cylinder chamber 20. The half angle a of the conical chamber 18should vary from zero to about l degrees. The conical chamber 18 shapeaffects the stability and the aerodynamic noise of the burner 10. Aproper conical chamber 18 angle provides an unobstructed passage for thecombustion products. The ratio of conical chamber 18 length (Lc) to thestep 16 diameter (Ds) should be in the range of 1.5 to 3.0, depending onthe allowable block length. The shallow cylindrical chamber 20 diameter(Dc) should be equal to the outer diameter of the conical chamber 18.The cylindrical length (Lc) to the cylindrical diameter (Dc should varyfrom 0.2 to 0.75, again depending on the allowable block length. Theblock 12 length depends on the burner size and application.

The throat mix burner 10 of this invention is designed to operateprimarily on natural gas, but it can be adapted for use with fuels suchas propane, manufactured gas, or No. 2 fuel oil.

The burner 10 is a balanced pressure type which uses a ratio controldevice, such as a zero regulator 39, to maintain the desired air-fuelratio. With the direct spark plug 52 ignition available on this burner10, normally it does not need any premix pilot connection.

The burner 10 can be operated over a wide range of air-fuel ratios. Thelimit on air-fuel ratio varies slightly for different burner sizes, butwith lean operation an air-fuel mixture of up to 70 percent excess airis achievable, and with rich operation, up to 90 percent excess fuel isnormal. This range of operation covers most major fields ofapplications, especially where the fuel efficiency and controlledatmosphere conditions are to be satisfied simultaneously. In the rangeof air-fuel ratios previously given, the burner 10 has a turndown ratioof 10 to l. The flame remains stable and gives a strong ultravioletsignal to the detection unit 56 at all firing rates. This wide range ofturndown allows considerable flexibility of furnace operation. It allowsthe use of fewer high capacity burners, resulting in low initial costson burners, piping and wiring. Using radial openings 48 in the nozzle44, the flame length is short and provides a high degree of circulationin a furnace. This circulation in a furnace helps to maintain uniformfurnace atmosphere temperature, and improves convection heat transfer tothe work.

The burner I0 is direct spark ignited at low firing rate, which isnormally one-tenth of the rated capacity. If necessary, a premix pilot,or even manual ignition, can be used with this burner 10. An ultravioletflame supervision system 56 can be used to monitor the flame at allfiring rates within the operational range of the burner. Due to theunique flame stabilization associated with the burner 10, the flamesignal strength, as represented by the flame detector output, remainsalmost constant at all firing rates. The location of the supervisionsystem insures that there is no interference between two or more burnersin a multi-burner arrangement in a furnace.

The burner operations were tested under both positive and negative backpressures in a furnace. It was found that a positive back pressure up to8 osi and a negative pressure down to 8 osi do not affect the flamestability or other characteristics of a burner 10 having a ratedcapacity of 0.25 MM BTU/hr. It is obvi ous that a proper correction forthe air and gas supply pressures should be allowed when the burner 10 isused under off-atmospheric back pressure. The same precaution appliesfor the burner 10 operations at elevated pressures.

Extensive testing was carried out to check the noise level and N0 CO andhydrocarbon emissions from the burner 10. The noise level of the burneris considerably lower than that from similar commercial burners. The N0CO and hydrocarbon levels depend on the furnace temperature. Laboratorytest results at furnace temperatures varying from 1,000F. to 2,950F.show that the concentration of N0 CO and hydrocarbon is much lower thanthe existing allowable concentrations.

The throat mix burner 10 can be used for most of the general purposeheating applications. Its operation characteristics, as discussedearlier, makes it attractive to furnaces used in ferrous and nonferrousmetal treatment, ovens, dryers, incinerators, steam generators, etc. Itcan be used on batch type or continuous furnaces. In either case, thehigh turndown capacity allows the use of fewer high capacity burners,contrary to the use of a large number of small capacity burners. Thepossible savings in initial cost of burners, piping, flame supervisionsystem, and the maintenance cost during the life of the burner aresubstantial.

What is claimed is:

l. A burner operable by a source of air and fuel comprising:

a block of refractory material having a preconflgured bore extendingtherethrough, said bore defined by a first cylindrical chamber at oneend of the block, a second larger cylindrical chamber at the oppositeend of said block and an intermediate connecting frusto-conical chambertherebetween, said frustoconical chamber having its large diameterportion equal to and adjacent said second chamber and its small diameterportion adjacent to and greater than said first chamber to define a stepat the juncture between said first and second chambers;

a hollow tubular member having a closed end section within said firstchamber, said end section being coaxially positioned within said firstchamber to define an annulus between said member and said first chamber,said tubular member having a plurality of radially directed openingsextending theret'hrough in said end section;

ignition means in fluid communication with said bore and operable toignite said burner;

gas means operable to supply said gaseous fuel within said tubularmember and through said opening into said annulus;

air means operable to supply combustion air under pressure through saidannulus effective as a jet pump to draw said gaseous fuel along withsaid air through said first chamber, to recirculate at said step saidfuel and air into a mixture sufficient to support combustion from saidignition means and to stabilize the products of combustion from saidrecirculated mixture at said step; the ratio of the diameter of saidstep to the diameter of said first chamber is in the range of [.5 to 3.0and the ratio of the first chambers axial length to its diameter is 0.5to 10.0; and the ratio of the second chambers axial length to itsdiameter is 0.2 to 0.75.

1. A burner operable by a source of air and fuel comprising: a block ofrefractory material having a preconfigured bore extending therethrough,said bore defined by a first cylindrical chamber at one end of theblock, a second larger cylindrical chamber at the opposite end of saidblock and an intermediate connecting frusto-conical chambertherebetween, said frusto-conical chamber having its large diameterportion equal to and adjacent said second chamber and its small diameterportion adjacent to and greater than said first chamber to define a stepat the juncture between said first and second chambers; a hollow tubularmember having a closed end section within said first chamber, said endsection being coaxially positioned within said first chamber to definean annulus between said member and said first chamber, said tubularmember having a plurality of radially directed openings extendingtherethrough in said end section; ignition means in fluid communicationwith said bore and operable to ignite said burner; gas means operable tosupply said gaseous fuel within said tubular member and through saidopening into said annulus; air means operable to supply combustion airunder pressure through said annulus effective as a jet pump to draw saidgaseous fuel along with said air through said first chamber, torecirculate at said step said fuel and air into a mixture sufficient tosupport combustion from said ignition means and to stabilize theproducts of combustion from said recirculated mixture at said step; theratio of the diameter of said step to the diameter of said first chamberis in the range of 1.5 to 3.0 and the ratio of the first chamber''saxial length to its diameter is 0.5 to 10.0; and the ratio of the secondchamber''s axial length to its diameter is 0.2 to 0.75.